1. Technical Field
This invention relates in general to a spectrophotometric system for non-invasively determining oxygenation levels in human tissue utilizing near-infrared spectrophotometric techniques, and in particular to such a system having various types of indicators which provide the user with information relating to the determined oxygenation levels.
2. Background Information
The molecule that carries oxygen in the blood is hemoglobin. Oxygenated hemoglobin is known as oxyhemoglobin (HbO2) and deoxygenated hemoglobin as deoxyhemoglobin (Hb). Total hemoglobin is the sum of the two states of hemoglobin (Total Hb=HbO2+Hb), and is proportional to relative blood volume changes, provided that the hematocrit or hemoglobin concentration of the blood is unchanged. The mammalian cardiovascular system comprises a blood pumping mechanism (the heart), a blood transportation system (blood vessels), and a blood oxygenation system (the lungs). Blood oxygenated by the lungs passes through the heart and is pumped into the arterial vascular system. Under normal conditions, oxygenated arterial blood comprises primarily oxyhemoglobin. Large arterial blood vessels branch off into smaller branches called arterioles, which profuse throughout biological tissue. The arterioles branch off into capillaries, the smallest blood vessels. In the capillaries, oxygen carried by hemoglobin is transported to the cells in the tissue, resulting in the release of oxygen molecules. Under normal conditions, only a fraction of the oxyhemoglobin molecules give up oxygen to the tissue, depending on the cellular metabolic need. The capillaries combine together into venuoles, the beginning of the venous circulatory system, and the venuoles combine into larger blood vessels called veins. The veins further combine and return to the heart, and venous blood is pumped to the lungs. In the lungs, deoxyhemoglobin collects oxygen thereby becoming oxyhemoglobin again and the circulatory process is repeated.
The amount of oxygen saturation is typically defined as the ratio of oxyhemoglobin to the sum of oxyhemoglobin and deoxyhemoglobin. In the arterial circulatory system under normal conditions, there is a high proportion of HbO2 to Hb, resulting in an arterial oxygen saturation (commonly referred to as SaO2%) in the range of 95-100%. After delivery of oxygen to tissue via the capillaries, the proportion of HbO2 to Hb decreases such that the measured oxygen saturation of venous blood (commonly referred to as SvO2%) is typically lower (e.g., 60%).
One common spectrophotometric method known as pulse oximetry determines arterial oxygen saturation of peripheral tissue (e.g., the finger, ear or nose) by monitoring pulsatile optical attenuation changes of detected light induced by pulsatile arterial blood volume changes in the arteriolar vascular system. Pulse oximetry requires pulsatile blood volume changes to make a measurement. Since venous blood is not pulsatile, pulse oximetry cannot provide any information about venous blood. Also, it is difficult to detect arterial pulse within the brain tissue itself by optical non-invasive means, which reduces the usefulness of pulse oximetry techniques in those applications.
Near-infrared spectroscopy (NIRS) is an optical spectrophotometric method that can be used to continuously monitor tissue oxygenation levels without use of pulsatile blood volume changes. The NIRS method is based on the principle that light in the near-infrared range (700-1000 nm) can pass easily through skin, bone and other tissues where it encounters hemoglobin located mainly within micro-circulation passages; e.g., capillaries, arterioles, and venuoles. Hemoglobin exposed to light in the near-infrared range has specific absorption spectra that vary depending on its oxygenation state; i.e., oxyhemoglobin and deoxyhemoglobin each act as a distinct chromophore. By using light sources that transmit near-infrared light at specific different wavelengths, and by measuring changes in transmitted or reflected light attenuation, concentration changes of the oxyhemoglobin and deoxyhemoglobin can be monitored. The ability to continually monitor cerebral oxygenation levels, for example, is particularly valuable for those patients subject to a condition in which oxygenation levels in the brain may be compromised, leading to brain damage or death.
NIRS-type sensors typically include at least one light source and one or more light detectors for detecting reflected or transmitted light. The light signal is created and sensed in a part of an overall NIRS system that includes a monitor portion having a computer or processor that runs an algorithm for processing signals and the data contained therein. Typically the monitor portion is separate from the sensor portion. As such, the sensor and monitor portions comprise the overall NIRS system. Light sources such as light emitting diodes (LEDs) or laser diodes that produce light emissions in the wavelength range of 700-1000 nm are typically used. A photodiode or other light detector is used to detect light reflected from or passed through the tissue being examined. The NIRS system processor cooperates with the light source and detector to create, detect and analyze the signals in terms of their intensity and wave properties. U.S. Pat. Nos. 6,456,862, and 7,072,071, both of which are hereby incorporated by reference in their entirety and are commonly assigned to CAS Medical Systems, Inc., of Branford, Conn., both disclose an NIRS system (e.g., a cerebral oximeter) and a methodology for analyzing the signals within the NIRS system.
It is known that relative changes in the concentrations of HbO2 and Hb can be evaluated using NIRS apparatus which may include a processor programmed to utilize, for example, a variant of the well-known Beer-Lambert Law, which accounts for optical attenuation in a highly scattering medium such as biological tissue. While this approach to determining oxygenation levels has some utility, it is limited somewhat in that the information it provides relates to a change in the level of oxygenation within the tissue. This approach does not provide for the total value or the absolute value of the oxygen saturation within the biological tissue.
It is known to utilize information regarding the relative contributions of venous and arterial blood within tissue examined by NIRS, where such information was either arbitrarily chosen or estimated, or was determined by invasive sampling of the blood as a process independent from the NIRS examination. For example, it has been estimated that NIRS examined brain tissue comprises about 60% to 80% venous blood and about 20% to 40% arterial blood by volume in the microvasculature. Regarding invasive techniques, blood samples from catheters placed in venous drainage sites such as the internal jugular vein, jugular bulb, or sagittal sinus have been used to evaluate NIRS measurements. However, the estimation technique and the invasive technique have obvious drawbacks, primarily relating to accuracy and to the invasive nature, respectively.
A distinct improvement over these prior art techniques for determining the level of oxygen saturation is the NIRS method and apparatus described and illustrated in the aforementioned U.S. Pat. Nos. 6,456,862 and 7,072,071. U.S. Pat. No. 6,456,862 describes an apparatus and a method for determining the total blood oxygen saturation within tissue. U.S. Pat. No. 7,072,071 also describes an apparatus and a method for determining absolute values of blood oxygen saturation within tissue.
It is further known in the prior art to use comparative spectroscopy methods, such as those described and illustrated in U.S. Pat. Nos. 6,615,065 and 5,873,821. Such methods typically utilize NIRS systems having two or more sensors located, for example, on the human head to access brain tissue with infrared light to thereby determine the oxygenation levels within the brain tissue. However, drawbacks with these comparative spectroscopy methods typically include the need to compare an oxygenation measurement of one region or hemisphere of the brain to oxygenation measurements of other regions of the brain to determine adverse physiological changes by differential analysis or by measuring differential changes from a predetermined initial baseline. Further, with comparative spectroscopy a clinician typically must wait for measurements to be different between two or more sensors to determine if a potential risk of brain damage exists. Therefore a particular disadvantage of comparative spectroscopy is that potential brain damage indications may be missed, because measurements from two or more sensors may give a similar value in which a differential value may be near zero.
What is needed is a spectrophotometric system that determines, for example, the total and absolute oxygen saturation levels within certain biological tissue (e.g., the brain) and provides for various types of indicators (e.g., visual and audible) to quickly and accurately convey to the system user information regarding, for example, the total and absolute oxygen saturation levels with respect to the human subject being monitored by the system.